Most optical scanning applications use a moving mirror, either rotating or oscillating. A laser beam is often projected onto the moving mirror to scan the beam across a specified linear or two-dimensional (2D) (raster) pattern at a frequency that is sufficient for the particular application. For optical displays, the field of view (FOV) is determined by the scanning amplitude and the particular optical design. There is a minimum frequency (rate) at which scanning displays need to be refreshed, which is determined by the human perception of flicker from a scanned display. For ubiquitous raster scanning displays, such as cathode ray tubes (CRTs) used in televisions and computer monitors, the display refresh rate is typically 30 to 60 Hz. Although a CRT employs an electron-beam for scanning an electro-optical display screen, the same requirements for scan frequency and amplitude (that determine the FOV) generally apply for all types of scanning displays. Thus, for a super video graphics array (sVGA) display having a CRT resolution of 800×600 pixels, the minimum horizontal scan rates are 40 kHz for unidirectional and 20 kHz for bidirectional scanning.
Combining both high resolution (>400,000 pixels) and wide FOV (>30°) in a single display is a difficult technical challenge, limiting the application of optical scanning for small size, low cost optical scanners that have both high-resolution and wide FOV. There is a tradeoff between optical scanning frequency versus scanning amplitude (FOV) for all mirror-scanning devices. The faster the mirror scans, the greater the forces acting on the mirror, which deforms the mirror surface, degrading image quality. This limitation is especially true for the small, low cost resonant mirror scanners. Rotating polygon mirror scanners can overcome this limitation or tradeoff between scan frequency and amplitude, except they are usually bulky, noisy, and costly. In the case of a resonant mirror scanner, the mirror cannot scan more than a few degrees in amplitude at frequencies of 20 to 40 kHz, as required for sVGA raster scanning displays. Since the optical beam reflects from the scanning mirror, the optical FOV is twice the total mirror deflection angle (i.e., the FOV=2 times mirror scan amplitude). However, at sVGA resolution and scan frequencies, optical FOVs on the order of 30° to 60° cannot be achieved using a low cost resonant mirror scanner as the basis for micro displays.
Recently, resonant mirror optical scanning systems have been developed that include silicon micro-machining techniques to make micro-electromechanical systems (or MEMS) devices. In theory, this technique can manufacture durable mirror-based optical scanners at lower costs. Nonetheless, there is still a tradeoff between scan amplitude and scan frequency of the resonant scanning mirror versus resolution. In practice, the relatively high capital investment required for creating a MEMS fabrication facility is a barrier for most companies. To date, a mirror-based resonant scanner fabricated as a MEMS device has yet to be demonstrated as a viable method for manufacturing low cost optical scanners for visual displays of wide FOV and at video scan rates.
There is a growing market for micro-optical displays as well as small optical sensors, optical switches, and scanning image acquisition systems. For example, a low cost micro-optical scanner is essential for spectacle-mounted, retinal light scanning displays and micro-displays that may be embedded in future cellular telephones. Moreover, there is a commercial need for low cost, large-scale (panoramic) optical displays, because larger CRT displays are uneconomical in energy and space. There is also a growing market for optical sensing and switching, especially in conjunction with fiber-optic sensing and communication applications. Finally, the lack of low cost micro-optical scanners with a wide FOV has been the most significant barrier for reducing the size of scanning image acquisition systems for use in surveillance, industrial inspection and repair, machine and robotic vision systems, micro-barcode scanners, and minimally-invasive medical imaging (flexible endoscopes).